So far, as a pumping device for liquid containers, there have widely been used various kinds of bellows pumps which serve to pump air into an interior of a container to increase its internal pressure, thereby pouring liquid such as hot or cold water in the container therefrom through a pumping tube. For example, as shown in FIG. 7, a bellows pump 100 is applied to a vacuum bottle 1 comprising a vacuum insulated, double walled inner bottle 7a housed in an outer container 7 with a lid body 2.
Such a bellows pump 100 generally comprises a bellows body 101 with upper and lower plates 102, 104 having a suction port 103 or an exhaust port 105, a suction valve 106 adapted to open and close the suction port 103, and a bellows return spring 107 arranged in the bellows body 101 to keep it expanded.
In use, the bellows pump 100 is operated by manually pushing down the push cover 5 to feed air into an interior of the inner bottle 7a. By manually pushing down the cover 5, the suction valve 106 is firstly closed, and the bellows body 101 is deflated so that the air in the bellows pump 100 is fed into the interior of the inner bottle 7a through the exhaust port 105 to increase the internal pressure of the inner bottle 7a. Thus, the liquid, such as hot water or cold water for example, in the inner bottle 7a is poured therefrom through a pumping tube 108.
As a rule, the bellows pump 100 involves a chamber of variable volume, having the maximum volume sufficient to fill a cup with the liquid in the inner bottle 7a. When it is required to pour a large amount of the contained liquid, the cover 5 has to be pushed down repeatedly, resulting in a troublesome operation. For example, when the cover 5, after being pushed down to its lowest position, is released from the external pressure, the suction valve 106 is forced upwardly to open the suction port 103. Thus, the increased pressure in the inner bottle is reduced to the atmospheric pressure every return stroke of the bellows pump 100, resulting in lowering of the pumping efficiency of the bellows pump. This becomes a large problem when pouring a large amount of a cold liquid. If the air is fed to the inner bottle containing cold water, the internal pressure of the inner bottle is not so increased since the air in the bottle is contracted by the cold heat of the liquid.
Further, the internal pressure of the inner bottle is increased every contraction of the bellows pump, so that the liquid, for example, hot water in the inner bottle is intermittently poured into hot water previously poured into the cup, resulting in scattering of the hot water.